1. Field of the Invention
This invention relates to capacitors in general and, more particularly, to high-voltage capacitors integratable with other circuitry on a common substrate.
2. Description of the Prior Art
For an increasing number of applications, such as for power supply and telephony applications, coupling capacitors are needed that can sustain high voltages (e.g. 1000 volts or greater) while providing moderate amounts of capacitance. Preferably, the capacitors are integrated along with other circuitry on a common silicon substrate.
Most monolithic capacitors, fabricated using existing integrated circuit processing technology, have a breakdown voltage limited to about 1500 volts. For some applications this may be insufficient to meet circuit requirements. To be able to tolerate voltages higher than the breakdown voltage of a single capacitor, two or more monolithic capacitors are placed in series. Thus, the total voltage impressed across the capacitors is the sum of the voltages across each capacitor in series. In theory, the voltage across each capacitor is less than the breakdown voltage thereof. However, if these is any leakage current through one of the capacitors, the voltage across that capacitor will tend toward zero, forcing a higher voltage across the remaining capacitor(s). The higher voltage may be greater than the remaining capacitor(s) can handle, causing a catastrophic breakdown in the remaining capacitor(s).
Intentional leakage across each capacitor(s) is sometimes used to force equalization of the voltages across the individual capacitor(s). A fixed resistor is coupled across each series capacitor to provide the intentional leakage. The amount of resistance is sufficiently low such that the leakage current therethrough is much greater than the amount of expected leakage through the corresponding capacitor. The intentional leakage overcomes the effect of the leakage through the capacitor. This solution may not be acceptable because the amount of intentional leakage may be more than what can be tolerated in a particular application. In addition, adding resistors to each capacitor may not be possible nor desirable depending on the topology of the capacitor.
Thus, it would be desirable to provide a monolithic capacitor that does not require a leakage resistor when disposed in series with other similar capacitors in high-voltage applications.